1. Field of the Invention
The invention relates to a semiconductor device, particularly to a diode preventing a parasitic current.
2. Description of the Related Art
Conventionally, a diode element has been generally provided on a semiconductor substrate. A diode is widely used as a simple resistor, a part of a constant-voltage circuit for obtaining a predetermined voltage or the like.
A conventional diode element provided on a semiconductor substrate will be described referring to figures. FIG. 10 is a plan view of the conventional diode element, and FIG. 11 is a cross-sectional view thereof along line X-X. An anode wiring 104 and a cathode wiring 106 in FIG. 11 are omitted in FIG. 10.
An N well layer 101 made of an N-type impurity is formed in a P-type semiconductor substrate 100. Element isolation insulation films 102a, 102b and 102c are formed, so as to define a corresponding square, on a predetermined region of the N well layer 101, on the boundary of the P-type semiconductor substrate 100 and the N well layer 101 and on the P-type semiconductor substrate by the LOCOS (Local Oxidation of Silicon) method or the STI (Shallow Trench Isolation) method.
A P+ layer 103 made of a P-type impurity is formed on the N well layer 101 surrounded by the element isolation insulation film 102a. The P+ layer 103 is connected with an anode electrode Anode through the anode wiring 104, and used as an anode region of the diode element.
An N+ layer 105 for a contact made of an N-type impurity is formed on the N well layer 101 between the element isolation insulation film 102a and the element isolation insulation film 102b. The N+ layer 105 is connected with a cathode electrode Cathode through the cathode wiring 106, and used as a cathode region of the diode element.
A P+ layer 107 made of a P-type impurity is formed on the front surface of the semiconductor substrate 100 on the outside of the element isolation insulation film 102b. The semiconductor substrate 100 is grounded through the P+ layer 107. The relevant technology is described in Japanese Patent Application Publication No. 2002-198436, for example.
However, when a forward voltage is applied to the described diode element to flow a current, as shown in FIG. 11, a vertical parasitic bipolar transistor 110 turns on by the P+ layer 103 serving as an emitter region, the N well layer 101 serving as a base region and the P-type semiconductor substrate 100 serving as a collector region, thereby causing a problem that a leakage current flows into the semiconductor substrate 100. This problem further causes various problems that low current consumption or a desired integrated circuit (e.g. a constant-voltage circuit) is not achievable, or the like.
The invention is directed to a semiconductor device having a diode element which reduces a leakage current to a semiconductor substrate and improves forward current capability.